Generally, a decalcomania (decal) is comprised of a multilayer structure including a backing paper, a design of pigment layer formed from inorganic pigments or oxides, and optionally a protective layer applied over the design layer. A layer facilitating release of the backing from the design may be interposed between the backing paper and the design layer. Such a decalcomania, depending upon its specific structure and especially the manner in which the backing paper is released from the design layer, can usually be classified as being of the slide-off type or the heat-release type.
The heat-release type of decalcomania includes a backing sheet which carries a heat-releasable wax layer, as will be described in detail hereinafter, which is separated from the backing sheet by means of a so-called barrier layer. A design layer including a resinous carrier is printed on the wax layer and a layer of heat-activatable adhesive is disposed on the design layer. Examples of typical heat-releasable decalcomanias are disclosed in U.S Pat. Nos. 2,970,076 to Porth; 3,007,829 to Akkeron; 2,991,221 to Bower; 3,411,458 to Milliken; 3,445,309 to Milliken and 3,642,551 to Laconich et al.
In using the heat-release type of decalcomania, the article to be decorated is preheated and the decalcomania is applied to the article with the backing sheet up so that the layer of heat-activatable adhesive directly contacts the article. The heat of the article effects both a preliminary bonding of the design layer, via the heat-activatable adhesive, to the article, and also the release of the backing sheet including the barrier layer and a portion of the wax layer, from the design layer. Thereafter, the article is fired at a high temperature in the usual manner so that the design layer is melted and permanently fused to the article; the combustible organic ingredients including the remaining portion of the waxlayer, the resinous carrier for the design layer as well as the heat-activatable adhesive layer are consumed during the firing.
As indicated, the technique for bonding heat-releasable type of decalcomanias requires that the article to be decorated must be preheated in order to soften the heat-activatable adhesive layer. The preheating must be carried out so that the area of the article to which the decalcomania is to be applied is maintained at a steady even temperature. Such preheating requires the use of expensive heating equipment and even with such equipment it is difficult to ascertain whether the article is heated to the required temperature. If, in fact, the article is not heated to the proper temperature, that is at the temperature required to soften the adhesive without causing it to melt and flow away, blisters or air pockets could form in the decalcomania as it is fired on the ware. This could, of course, ruin the decorative effect of the decalcomania.
Preheating of the articles has also been found objectionable to personnel handling the articles. The preheated articles are rather hot and personnel handling same usually are required to wear bulky and cumbersome equipment to protect them from being injured.
It should also be noted that these various decalcomanias have been employed for the decoration of various articles, including glassware, ceramic ware, etc. In connection with said decoration of ceramic ware, decalcomanias of both the "underglaze" and "overglaze" type have been utilized. In the former case, the decal is applied to the ware after its formation, but prior to its being glazed, which is accomplished after application of the decal to the ware. The glaze employed comprises a vitreous coating which generally requires a very high temperature to form the glass from its raw materials. While this procedure therefore results in a protective coating over the pigments in the decal, these high temperatures generally destroy the color values of these pigments. Thus, "overglaze" decalcomanias have also been developed, i.e. -- which are applied to the ware after it has been glazed. These generally include silk screen and lithographic decals. The present invention is intended for application in connection with all of the various types of decal applications.
In addition, in the past various adhesive films have been developed for use on labels, adhesive tapes, and the like which have a property referred to as "delayed track". This refers to the characteristic of these adhesives such that a thermoplastic composition, after being heated to a temperature at which it becomes tacky or adhesive, thereafter remains in its tacky condition for an interval after heating is discontinued. Such adhesive compositions are taught, for example, in U.S. Pat. No. 2,462,029 to Perry; U.S. Pat. No. 2,613,191 to McGaffin et al; U.S. pat. No. 2,746,885 to Holt, Jr.; U.S. Pat. No. 2,608,543 to Wiswell; and U.S. Pat. No. 2,613,156 to McGaffin et al. Each of these patents relates to such adhesive compositions for use in labels and the like including the use of plasticizers in connection with various thermoplastic polymers in aqueous solutions. Furthermore, in many of these cases, the thermoplastic materials are used as thickeners or binders, containing inorganic salts and the like.